As Wisconsin Gov. Scott Walker watches his presidential poll numbers sink lower and lower, he can take some solace in this:

He's not alone.

Walker has fallen the furthest. But other major contenders such as former Florida Gov. Jeb Bush and Sen. Marco Rubio of Florida have also been left in the dust by real estate baron Donald Trump and retired neurosurgeon Ben Carson.

Are there second acts for struggling presidential hopefuls?

Can a sustained polling slump like the ones suffered this year by Walker, Bush and Rubio be overcome?

History doesn't offer many examples (partly because there was a lot less early polling in the past).

But the most tantalizing one may be Republican John McCain, whose campaign collapsed financially in 2007 and was presumed dead by many before the senator from Arizona roared back to life and won the nomination.

McCain lost about 15 points in the New Hampshire polls between the spring and fall of 2007, but then rode a 20-point surge to victory there.

"It's hard, but the McCain example demonstrates it's not impossible," said a GOP strategist who worked for McCain and did not want to be quoted by name. "Whoever does this is going to be a very skilled politician."

In McCain's case, it took mistakes by opponents, a core of die-hard supporters dating back to his first presidential bid, a bond with the early primary state of New Hampshire and the "capacity to drag a dead campaign forward on the strength of your personality," the strategist said.

Whether Walker can find a similar formula is a real question.

History offers far more examples of candidates who couldn't survive a polling plunge. But there's a decent chance this race will produce a true comeback story of some kind.

If the Republican nominee turns out to be Walker, Bush or Rubio — hardly a long-shot scenario even now — that will mean somebody bounced back from months of poor polling.

"I don't think there's any particular reason for (Walker's camp) to be optimistic, but I also don't think there's a need to be overly pessimistic," said Larry Sabato, a University of Virginia political scientist and founder of the school's Center for Politics. "I think Walker's consolation is he's got a lot of company in the Republican field scratching their heads saying what happened and what's going on?"

Walker and his backers argue that it's still early, and that most 2015 polls are reflecting shifts in media coverage and attention, not hard voter preferences.

Many scholars agree.

"Even politically engaged people who are going to vote in the primaries are not necessarily paying attention," says Georgetown University political scientist Hans Noel, co-author of an influential study of the nominating process, "The Party Decides."

Noel said if he were Walker, he wouldn't lose hope because of the polls. But he said he would be deeply frustrated about the suffocating effects of the Trump phenomenon on his ability to reset his campaign.

Walker has downplayed the polls, noting he won his 2012 recall election despite a slump a year before then.

"Heck, I was so far down in the polls in the spring of 2011, Time magazine called me Dead Man Walker," the governor told reporters Thursday. "I came back and won the recall with more votes...than we did in the original election. So none of this intimidates us."

To some observers, history may not be all that instructive about Walker's plight, since this race seems different in significant ways: the massive, 16-candidate GOP field; the voracious appetite of Republican voters for a political outsider; and changes in the length and nature of the "invisible primary" (the early campaigning that occurs before the first caucus or primary). The presence of super PACs funded by rich individuals means candidates can more easily survive a polling slump that would have dried up their funding in the past. That made it possible in 2011 for Republican Newt Gingrich to rebound from a major polling slide before the first contest — only to collapse again.

In this race, Trump and Carson together — two men who've never held office — are claiming more than 40% of the GOP electorate in many polls.

In an Iowa survey released by Quinnipiac Friday, 72% of Republicans said the best preparation for being president was working in business, not government.

Normally, people without experience in elected office can't maintain early success in presidential contests, said Charles Franklin of the Marquette University Law School. As examples, he cited Pat Robertson in 1988 and Steve Forbes in 1996 and 2000.

"There's still a chance for a major reshuffling," Franklin said. "Let's not forget: We're one debate in, with — what? — eight debates to go."

While Walker's numbers have tanked, Franklin noted that all the candidates with experience in office have either lapsed into single digits or have been mired there to begin with.

In the national polling averages posted by RealClearPolitics.com, Walker has slipped from a peak of just over 17% on April 1 to just under 5% last week. Bush has slipped from a peak of nearly 18% in mid-July to just over 8% last week. And Rubio has slipped from a peak of just over 14% in early May to just over 5% last week.

Walker's slide has been even sharper in Iowa, where he was once the clear front-runner. In Quinnipiac's new Iowa survey, he has fallen to 3% from 25% in February and 18% in July. But his personal image among Iowa Republicans remained quite positive over that time. The numbers suggest Walker has been eclipsed more than rejected by Iowa Republicans: Only 5% said they could not support him, and only 15% have an unfavorable opinion of him (62% have a favorable view). Yet for Walker to derive any benefit from his positive standing in the state, Trump and/or Carson will have to falter.

"You're in a position where you don't really control your fate," the former McCain strategist said of Walker, Bush and Rubio. "You're just waiting for the bumper cars to collide and leave enough slots open to get considered again."

It's possible that no one is able to sustain himself or herself as the clear front-runner, said Jennifer Duffy, senior editor of the Cook Political Report.

"I'm of the school that we might have four different winners for the first four contests," she said. "So then you go, 'Who was smart enough to put something together in the South?'"

The first four states — Iowa, New Hampshire, South Carolina and Nevada — have very different electorates, meaning a candidate who is strong in one state may not be in another. Iowa and Nevada hold caucuses, rather than primaries, so their contests require intense organizing by candidates.

After that is Super Tuesday on March 1, when 13 states — eight of them in the South — hold primaries and caucuses.

Jason Stein of the Journal Sentinel staff contributed to this report.

About Craig Gilbert

Craig Gilbert is the Journal Sentinel's Washington Bureau Chief and writes the Wisconsin Voter blog about politics and elections.

About Patrick Marley

Patrick Marley covers state government and state politics. He is the author, with Journal Sentinel reporter Jason Stein, of "More Than They Bargained For: Scott Walker, Unions and the Fight for Wisconsin.”